1. Field of the Invention
This invention relates to digital communication, and more specifically to digital voice communication.
2. Discussion of Related Art
High quality reproduction of speech signals at low bit rates is extremely important for a system with valuable fixed resources, such as a fixed bandwidth. Improvements in voice coding, vocoding, that reduce bit rate while maintaining speech quality can dramatically increase the total number of users on a fixed bandwidth, and therefore increase revenue.
Linear predictive coding (LPC) has been shown to provide acceptable performance at rates below 2.4 kbps. In LPC, short-term changes in a voice patterns are reflected as changes in spectral envelope. The spectral envelope may be quantized using stored tables into Line Spectral Frequencies (LSF's) which are transmitted to a receiver for decoding. Efficient quantization of the spectral envelope, and therefore of the LSF's, are vital to producing high quality speech.
The LSFs are decoded at a receiver making use of stored tables which are the inverse of those used at the transmitter for quantization. Larger tables provide higher quality, but require a larger memory.
Currently there is a need for a lower rate digital voice communication technique exhibiting the same, or increased voice quality, with lower complexity and lower storage requirements.